1. Field of the Invention
This invention relates to a substituted benzene derivative used as a component of a liquid crystal and a liquid crystal composition containing the same, having superior characteristics.
2. Description of the Related Art
Display devices having applied liquid crystals are those utilizing an electrooptical effect based on the dielectric anisotropy and electroconductive anisotropy of liquid crystal substances. Liquid crystal display modes include various ones such as dynamic scattering mode, twist nematic mode, supertwist nematic mode, phase transition mode, DAP mode, guest-host mode, etc. Properties required for liquid crystal substances used for liquid crystal displays vary depending on the respective liquid crystal display modes, but a broad mesomorphic range, stability to moisture, air, light, heat, electricity, etc. are required for any of the display modes commonly employed. Further, it is also required that the response of display elements is rapid and the devices can be driven at a voltage as low as possible when the substances are used for liquid crystal display devices. At present, however, there is no single compound which satisfies all of such requirements, but there have been practically used liquid crystal mixtures obtained by mixing several kinds of liquid crystal compounds or mixing them with compounds similar to liquid crystals compounds.
Recently, in order to provide a liquid crystal display element having a good pictorial quality even in a multiplex number of 100 or more, it has been proposed to change the cell structure having the twist angle of the helical structure of liquid crystal molecule, combined with the polarizing plate to a new mode (e.g. Japanese patent application laid-open Nos. Sho 60-50511/1985, Sho 60-50454/1985, etc.)
The object of the present invention is to provide a liquid crystal compound suitable for a liquid crystal composition for liquid crystal display elements, particularly suitable to the above new mode structure.
The liquid crystal elements of a cell structure having an enlarged twist angle of liquid crystal molecules exhibits an entirely different effect in the values of physical properties obtained by choice of liquid crystal materials, from those in the case of conventional 90.degree. twist. Characteristics of liquid crystal display elements having an enlarged twist angle are shown in FIG. 1, as compared with those in the case of conventional 90.degree. twist. FIGS. 1a and 1b each show the viewing angle-dependency of voltage-transmittance characteristics in the cases of 90.degree. twist and 180.degree. twist. As seen from FIGS. 1a and 1b, the element having a 180.degree. twist structure is steep in the decay characteristics (.gamma. characteristics) of the transmittance by impressed voltage; thus it is evidently improved in the characteristics as compared with the case of conventional 90.degree. twist. This tendency is notable with an increase in the twist angle. As described above, since an element structure having an enlarged twist angle affords a steepness to the decay characteristics by impression of voltage, the difference between the transmittance by impression of voltage and that by non-impression of voltage at the time of multiplex drive becomes large, thus a higher multiplex drive than the conventional one becomes possible.
However, as to the relationship between the .gamma. characteristics in the case of 90.degree. twist and the .gamma. characteristics in the case of about 200.degree. twist, the same tendency is not exhibited depending on liquid crystal materials, but there has appeared a phenomenon which does not apply to the following report that, in general, the .gamma. characteristics in the case of 90.degree. twist are better when the ratio of the elastic constants (K.sub.33 /K.sub.11) is smaller (Euro Display 84 "Liquid crystal properties in relation to multiplexing requirements", Gunter Baur). This is presumed to be related to the change of the elastic constant and other factors due to the enlargement of twist angle, but it has not yet been theoretically elucidated. FIG. 2 shows the relationship between the ratio of the elastic constants and .gamma. characteristics in the case of 180.degree. twist and that in the case of 230.degree. twist, with two compounds i.e. a pyrimidine compound and a PCH (phenylcyclohexane) compound, respectively. As seen from this figure, the pyrimidine compound having a smaller elastic constant exhibits inferior results of .gamma. characteristics to those of the PCH compound. As described above, in the case of about 200.degree. twist, a conventional thought is not applied thereto; hence it is necessary to choose liquid crystal materials from an entirely different way of thinking from that in the case of 90.degree. twist cell structure. As described above, by enlarging the twist angle or selecting liquid crystal materials, a liquid crystal material having superior .gamma. characteristics is obtained. However, as shown in FIG. 3, by improving .gamma. characteristics, it has been found that the better the .gamma. characteristics of liquid crystal materials, the smaller the response rate. When the response properties are taken into consideration, choice of only materials having superior .gamma. characteristics is not always sufficient, but in order to obtain a liquid crystal display element having superior response properties, while retaining superior .gamma. characteristics to a certain extent, a method of reducing the thickness of the liquid crystal layer can be considered.
Thus, when the thickness of the liquid crystal layer is reduced in order to improve response properties, the .DELTA.n of the material should be varied according to the thickness of the liquid crystal layer. For example, in the case of 200.degree. twist, the product of the .DELTA.n of liquid crystal material by the thickness d of the liquid crystal phase (.DELTA.n.multidot.d) is best in the vicinity of 0.96 .mu.m; hence when the liquid crystal layer has a thickness d of 7 .mu.m, the .DELTA.n of the material should be adjusted to 0.137. Further, in order to make the thickness d of the liquid crystal layer 5 .mu.m for improving the response rate, it is necessary to increase the .DELTA.n value greatly up to 0.192. As described above, in order to correspond to the reduction in the thickness of the liquid crystal layer, the .DELTA.n of the material should be increased, but this case is accompanied with a problem of viscosity. As to the relationship between the .DELTA.n and the viscosity of liquid crystal materials so far reported, there has been clarified a tendency that the viscosity increases with increase in the .DELTA.n. In such a situation, among conventional materials, those having a large .DELTA.n value and yet a low viscosity have been very few.
As examples of compounds having a large optical anisotropy (.DELTA.n) value, used as a component of liquid crystal materials, compounds expressed by the following formulas (1)-(6) have been disclosed in the following references, respectively: ##STR2## wherein R.sup.3 and R.sup.4 each represent an alkyl group or an alkoxy group. ##STR3## wherein ##STR4## and R.sup.5 and R.sup.6 each represent a linear alkyl group. ##STR5## wherein R.sup.7 represents a linear alkyl group and X represents a halogen atom. ##STR6## wherein R.sup.8 and R.sup.9 each represent a linear alkyl group. ##STR7## wherein R.sup.10 and R.sup.11 each represent a linear alkyl group. ##STR8## wherein R.sup.12 and R.sup.13 each represent a linear alkyl group. (1) French patent application laid-open No. 2,141,438 (corresponding U.S. Pat. No. 3,925,482)
(2) Japanese patent application laid-open No. Sho 60-15427/1985 (corresponding U.S. Pat. No. 4,705,905) PA0 (3) Japanese patent application laid-open No. Sho 61-260031/1986 PA0 (4) Japanese patent application laid-open No. Sho 60-204731 (1985) (corresponding U.S. Pat. No. 4,705,905) PA0 (5) Japanese patent application laid-open No. Sho 58-110527 (1983) (corresponding to U.S. Pat. No. 4,528,114) PA0 (6) Ditto
These tolan derivatives are said to have a large .DELTA.n value and a relatively low viscosity, but recently, those far exceeding the characteristics of these compounds are required.